[llvm] [NFC][KeyInstr] Add (LLVM_)EXPERIMENTAL_KEY_INSTRUCTIONS (cmake/)definition (PR #131344)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

LGTM; this is how the RemoveDIs prototype was implemented, hopefully like RemoveDIs it'll avoid un-necessary interference with other folks while still getting test coverage.

https://github.com/llvm/llvm-project/pull/131344